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Marble Trebuchet

by Zarquon, published

Marble Trebuchet by Zarquon Nov 12, 2012

Description

This is a rough 1/8 scale model of my teams entry into Punkin Chunkin ( punkinchunkin.com/ ). There is a good deal of tuning which goes into a trebuchet and it's difficult to work it all out full scale since we only get some many shots between repairs. This model is to facilitate tuning work. Also just for fun. We enter purely for fun and our budget it way on the low side to be competitive but that's no excuse not to tune up.

At full scale the treb is 12 feet tall to the pivot and uses 1000lbs (1400 with add-ons) counterweight to throw 8 lbs pumpkins. Simulations show a maximum range of a bit over 800 feet. Our best throws are only a bit over 400.

At 1/8 scale it's about 18 in tall to the pivot, uses 2lbs counterweight and throws .25 ounce marbles. The first few days of throwing show a range of about 55 feet. The simulated max range is about 100 feet so there is tuning to do.

Give enough time I would like to add a more efficient whipper style arm to the model/machine.

Recent Comments

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It's been considered before but the
wheeled configuration is primarily for a fixed weight trebuchet.
When the weight can swing anyway you don't see much of an
improvement.

We're more concerned with keeping the
path the weight travels smooth and not intersecting with the frame
supports. When the whole frame is moving it's difficult to keep
square enough.
Not sure if it's feasible for the scaled up design, or if your competition allows it, but the addition of wheels may also help increase its range- pbs.org/wgbh/nova/lostempires/trebuchet/wheels.html

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Instructions

Print 2 of each part except the pivot.

Cut the following lengths of 5/16 (8mm) rod:
6x20in
4x18in
2x9in
3x4in

1x34.5in (hardwood for this one if available)

Other things you will need:
2x 608 bearings
1x5-6in 5/16 threaded rod
2-4 nuts for the threaded rod
eye screw with threads ground out for firing pin
finishing nail for sling release
String for rigging
pouch for sling
tent stakes to hold it down
2lbs of weight
.25 ounce projectiles (large marbles)

Some rods fit in tightly for me and others I had to glue. In any case you will want to at least glue the diagonal supports into the base corners and the bearings into the holders as those will pull out as the weight swings. The pivot will slide on the hardwood rod and should also be secured. The pivot should be located 24in from one end of the arm. The sling length should be 21 in.
Not sure if it's feasible for the scaled up design, or if your competition allows it, but the addition of wheels may also help increase its range- pbs.org/wgbh/nova/lostempires/trebuchet/wheels.html
It's been considered before but the
wheeled configuration is primarily for a fixed weight trebuchet.
When the weight can swing anyway you don't see much of an
improvement.

We're more concerned with keeping the
path the weight travels smooth and not intersecting with the frame
supports. When the whole frame is moving it's difficult to keep
square enough.
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